Process water associated with and produced by phosphate manufacturing operations is typically acidic and typically contains various dissolved constituents such as fluoride, ammonia, silica, sulfate, calcium, heavy metals, phosphate, magnesium, colloidal matter, organic carbon, and, in some instances, radium (a radioactive element). Ponds associated with past phosphate processing contain billions of gallons of this waste water. There is an urgent environmental need to treat this wastewater, particularly in environmentally sensitive areas, or areas where population growth has come into closer contact with phosphate processing sites. Treatment of this waste to reduce its toxicity and its volume has been a technological challenge of significant interest. The toxic or harmful contaminants must be either reduced or eliminated before treated water can be discharged into the environment.
Various techniques have been used to reduce the level of such constituents before water is discharged. For example, double liming, followed by air stripping, can be used. This process adds lime in two stages, to promote precipitation of fluoride species and phosphate species, followed by high pH air stripping to remove ammonia. In another technique, water is treated by techniques involving chemical precipitation followed by reverse osmosis. Like double liming, such techniques raise the pH of influent water to promote precipitation and solids separation before reverse osmosis.
Reverse osmosis involves separating water from a solution of dissolved solids by forcing water through a semi-permeable membrane. As pressure is applied to the solution, water and other molecules with low molecular weight and low ionic charge pass through small pores in the membrane. Larger molecules and those with higher ionic charge are rejected by the membrane.
Some constituents that can be found in water, such as fluoride and phosphate, tend to form soluble acids under acidic conditions thus reducing the potential for scaling of reverse osmosis membranes. Other constituents that can be found in water, such as ammonia, tend, under acidic conditions, to form salts that are easily rejected by the membranes. In dual-pass reverse osmosis systems, the pH of permeate from the first pass reverse osmosis membranes can be adjusted upwards towards neutral conditions between the first and second pass membranes to make it easier to remove constituents that tend to exist in soluble form under highly acidic conditions.
Antiscalants can be added before first pass and/or second pass reverse osmosis membranes. Typically, antiscalants are materials that interfere with precipitation reactions by mechanisms such as crystal modification in which negative groups located on the antiscalant molecule attack the positive charges on scale nuclei interrupting the electronic balance necessary to propagate the crystal growth. Similarly, some antiscalants adsorb on crystals or colloidal particles and impart a high anionic charge, which tends to keep the crystals separated.
Some treatment systems include pretreatment before the reverse osmosis membranes to remove constituents such as suspended solids that can clog the reverse osmosis membranes. Some treatment systems include polishing technologies to reduce the residual concentrations of constituents for which allowable discharge concentrations are very low. Although these polishing technologies may be necessary to meet discharge criteria, they can add significantly to the overall treatment system operating costs.